feat: add native TTFT, TPOT, ITL, and E2E latency tracking to framework

[sats-23]

-Implements critical inference metrics directly within the IGW framework, removing the dependency on the SLO predictor plugin for observability.
-Framework now natively tracks

Time to First Token (TTFT)
Time to Predict Output Token (TPOT)
Inter-Token Latency (ITL) ( request_itl_seconds )
Decode Duration ( request_decode_duration_seconds ): Total Duration - TTFT
End-to-End (E2E) latency ( request_e2e_latency_seconds )

-Added tests to validate metrics tracking

What type of PR is this?
/kind feature

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #2163

Does this PR introduce a user-facing change?:

Metrics (TTFT, TPOT, ITL, Decode Duration, E2E latency) are directly exposed by IGW framework removing the dependency on SLO predictor plugin for basic metrics

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: sats-23
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[k8s-ci-robot]

Hi @sats-23. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[ahg-g]

/ok-to-test

[sats-23]

/test pull-gateway-api-inference-extension-verify-main